1. Field of the Invention:
This invention relates to a process for purifying hydrogen fluoride (HF). More specifically but not by way of limitation, the invention relates to the removal of trivalent arsenic (As.sup.+3) impurities from anhydrous HF by oxidation of As.sup.+3 to the pentavalent arsenic (As.sup.+5) using hexavalent chromium oxide (CrO.sub.3) and oxygen followed by distillation of the HF.
2. Description of the Prior Art:
Anhydrous hydrogen fluoride is formed by heating a mixture of fluorspar and sulfuric acid. The main impurities in the prepared hydrogen fluoride are flurosulfonic acid, silicon tetrafluoride, sulfur dioxide, sulfuric acid and water. These impurities are usually removed by fractional distillation, and the resulting anhydrous hydrogen fluoride has a purity of about 99.8% or better. However, the anhydrous hydrogen fluoride thus obtained still contains unacceptable quantities of undesirable impurities such as arsenic. The amount of arsenic impurity which is present in the anhydrous hydrogen fluoride depends on the amount of arsenic-bearing impurities in the fluorspar used to prepare the anhydrous hydrogen fluoride.
The anhydrous hydrogen fluoride generally contains about 20 to 600 part per million (ppm) of arsenic impurity but can be as high as 1500 ppm, again dependent upon the particular source of fluorspar. The presence of this impurity at these levels is undesirable for many applications.
The degree of purity of anhydrous hydrogen fluoride required is to a great extent dependent on the particular end use application. Thus it is generally known that for such applications as found in the electronics industry such as cleaning agents and etchants in the production of semiconductors, diodes and transistors, a high degree or purity and extremely low levels of impurities are required. Typically arsenic concentrations in terms of a few parts per billion are desirable. Thus the prior art discloses several anhydrous hydrogen fluoride purification processes intended to reduce the arsenic concentration to levels measured in terms of parts per billion. However, these known processes are characterized as involving a combination of costly reagents, equipment and/or procedure as well as frequently requiring prolonged periods of time.
For example, U.S. Pat. No. 3,166,379 discloses a method whereby high purity aqueous hydrogen fluoride is produced by treatment with an oxidizing agent, such as permanganate salts, in combination with a halogen, preferably iodine, to convert arsenic impurities to non-volatile compounds. This process is suitable for the purification of aqueous hydrogen fluoride but suffers from the drawback of contamination with volatile manganese compounds when anhydrous hydrogen fluoride is distilled from the aqueous solution.
A solution to this problem is claimed in U.S. Pat. No. 3,689,370 which describes a process involving the addition of an inorganic ferrous salt to the anhydrous hydrogen fluoride after the permanganate or dichromate treatment to reduce excess oxidizing agents. However, this process results in a high level of iron contamination.
The iron contamination problem is solved in U.S. Pat. No. 4,032,621 which describes a process involving the addition of a heavy-metal-free reagent, such as hydrogen peroxide, to anhydrous hydrogen fluoride after the permanganate or dichromate treatment to reduce excess oxidizing agent. This process is very sensitive and may not be convenient for continuous operation.
In U.S. Pat. No. 3,687,622 the distillation of impure anhydrous hydrogen fluoride containing 1200 ppm As at very high pressures (e.g., &gt;115 psia and preferably &gt;165 psia) is disclosed wherein the As is removed overhead, and purified hydrogen fluoride (e.g., &lt;3,000 ppb and preferably &lt;100 ppb As) is recovered as bottoms product. In U.S. Pat. No. 3,663,382 As impurities are removed from anhydrous hydrogen fluoride by distillation at a pressure below 25 psia, with purified hydroqen fluoride being recovered as the overhead product.
In East German Patent No. 62,309 (June 20, 1968) the removal of arsenic from 80-90% aqueous hydrogen fluoride with 30% hydrogen peroxide (H.sub.2 O.sub.2) at 40.degree.-60.degree. C. is disclosed. The reference also teaches that arsenic removal from anhydrous hydrogen fluoride can be accomplished by continuously adding anhydrous hydrogen fluoride and hydrogen peroxide solution of suitable concentration to the reactor maintained at 80-90% hydrogen fluoride and 40.degree.-60.degree. C. The reactor contents are distilled and absorbed in water to produce a purified aqueous solution of HF. This process is limited to production of aqueous solutions of HF product and is not suitable for the manufacture of purified anhydrous hydrogen fluoride.
U.S. Pat. No. 4,083,941 claims removal of arsenic and sulfite impurities from anhydrous hydrogen fluoride by the addition of 0.7% H.sub.2 O.sub.2 and 0.6% methanol at 0.degree.-75.degree. C. or by the addition of at least 2.3% by weight of persulfuric acid (H.sub.2 SO.sub.5) based on anhydrous hydrogen fluoride.
U.S. Pat. No. 4,491,570 claims arsenic removal from anhydrous hydrogen fluoride by treating with elemental chlorine and anhydrous hydrogen chloride or a fluoride salt, or both, and then separating purified anhydrous hydrogen fluoride by distillation from the non-volatile arsenic compounds. In U.S. Pat. No. 4,667,497 a process involving the addition of elemental fluorine to oxidize impurities present in hydrogen fluoride, followed by distillation, is disclosed.
U.S. Pat. No. 4,756,899 claims arsenic removal from anhydrous hydrogen fluoride by treating with hydrogen peroxide in the presence of a catalyst, which is comprised of molybdenum or an inorganic molybdenum compound, and a phosphate compound followed by distillation.
In contrast to the known ultra high purity processes of the prior art, technical or industrial grade anhydrous hydrogen fluoride containing typically from about 50 to 100 parts per million arsenic can usually be used in chemical processing or in the oil refining industry without too much difficulty. However, when the arsenic impurity level is higher, catalyst deactivation is usually accelerated and at very high arsenic levels (e.g., from about 200 ppm to about 1500 ppm) corrosion of processing equipment also becomes very severe. For example, in the process of fluorinating chlorocarbons with hydrogen fluoride in the presence of antimony halide catalysts to produce fluorinated hydrocarbons, arsenic in hydrogen fluoride will accumulate in the antimony halide catalysts thus contributing to the accelerated deactivation of the catalysts. When the deactivated catalyst is reactivated or discarded, the presence of large amounts of arsenic in the spent antimony halide catalyst presents handling problems. The presence of large amounts of arsenic in the processing system can lead to greatly accelerated corrosion of process equipment if an oxidant, such as chlorine, is also present.
In the commercial manufacturing of anhydrous hydrogen fluoride the technical grade hydrogen fluoride is purified by one or more final distillation steps. This conventional fractional distillation is effective in removing most of the major impurities except for the arsenic impurities. Usual distillation procedures are ineffective in significantly reducing the arsenic level in anhydrous hydrogen fluoride since arsenic is present in the trivalent (As.sup.+3) form as arsenic trifluoride which will codistill with hydrogen fluoride. Consequently there exists a need for a process that inexpensively and effectively decreases the arsenic impurities in anhydrous hydrogen fluoride to a level of at least less than about 100 ppm and preferably below 30 ppm.